Measurement of the $\ddot{I}‰\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}\ddot{I}{€}^0$ Dalitz plot distribution

Abstract

Using the production reactions $pd\to {}^3\mbox{He}\,ω$ and $pp\to pp\ddot{I}‰$, the Dalitz plot distribution for the $\ddot{I}‰\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}\ddot{I}{€}^0$ decay is studied with the WASA detector at COSY, based on a combined data sample of $(4.408\pm 0.042)\times 10^4$ events. The Dalitz plot density is parametrised by a product of the $P$-wave phase space and a polynomial expansion in the normalised polar Dalitz plot variables $Z$ and $\ddot{I}•$. For the first time, a deviation from pure $P$-wave phase space is observed with a significance of $4.1\ddot{I}ƒ$. The deviation is parametrised by a linear term $1+2\hat{I}\pm Z$, with $\hat{I}\pm$ determined to be $+0.147\pm 0.036$, consistent with the expectations of $\ddot{I}$-meson-type final-state interactions of the $P$-wave pion pairs.

8 pages, 7 figures